Crystal filter of variable band width



Oct. 15, 1940. H GOER G 2,218,087

CRYSTAL FILTER OF VARIABLE BAND WIDTH Filed Feb. 15, 1939 INVENTOR. HA 5 GOER/NG A TTORNE Y.

Patented Oct. 15, 1940 UNITED STATES GRYST'AL FILTER F VARIABLE BAND WIDTH Hans Goering, Berlin, Germany, assignor to Tclefunken Gcsellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application February 15, 1939, Serial No. 256,459 In Germany March 18, 1938 Claims. (01. 178-44) It is known in the art that the band width of crystal filters may be varied by. making the crystal the series element between two parallel resonance circuits connected as shunt elements, one of the two resonance circuits, or both of them, being detuned in reference to the frequency passed by the crystal or else by varying the coupling between the two oscillatory circuits and the crystal. At all events, the parallel capacitance of the crystal is' neutralized by a bridge scheme or else by connecting in parallel a coil dimensioned so as to insure resonance at the transmitted frequency. However, such arrangements inhere the drawback that the resonance curve is monowave (has only one hump) only for a limited band width, and that it exhibits two resonance points or two waves whenever the band width increases. The maximum band width which is possible is then limited by the admissible crevassing of the resonance curve. Now, this shortcoming is obviated by the present invention. In other words, the invention has the advantage that='for different band widths the characteristic form of, the resonance curve is preserved without it being necessary to replace the crystal itself. Ihe' characteristic shape of several resonance curves is the same if, for a definite ordinate value, the band widths of the various curves difier from one'another by the same factor as for any other ordinate value at all, in other words, that is, bear the same ratio. The slopes or sides of the curves, that is to say, the widths of the transition intervals, referred to the band width are equal to one another for a certain ordinate value.

The invention is concerned with a crystal filter which consists of a crystal and which forms the line or series element between two parallel.

resonance circuits connected as shunt arms or elements, the crystal being neutralized. According to the invention; to the end of increasing the band width, the ratio between the inductance and the capacity of the parallel resonance circuits being tuned to the pass-bandfrequency'for all band widths is raised, and to increaseat the same time the terminating resistance at both ends in such a way that the ratio of each of the terminating resistances to the increased surge impedance of the filter and thus also the characteristic form of the resonance; curve of the filter are preserved. 1

The invention is particularly useful in the I. F. part of superheterodyne receivers, for in the operation of these it is often necessary to make adjustments so that, at will,"very narrow resonance curves for telegraphic signal reception and broad resonance curves presenting steep flanks for telephonic reception can be received. The quartz crystal which is most important for practical use; is important in connection with quartz filters, by the way, not onlyfor thepurpose of obtaining very narrow or pointed resonance curves, but it is required, because of its low losses also so as to secure great steepness, that is, a narrow transition interval, for broad, resonance curves, since series resonance circuits for the line elements of RF filter circuits have heretofore not been designed with suchlo-w losses in any other manner. Fig. 1 shows the fundamental layout of the filter circuits. Fig. 2 is an exemplified embodij ment of the invention, while Fig. 3 shows resonance curves obtained with the filter of the invention.

Referring to Fig. LS is an A. C. source of current whose E. M. F. is denoted by E and its internal resistance by R1. This internal resistance in the present schemeserves as a terminating resistance of the input end of the filter. In-

ductance LI and capacitiesCi and C5 represent the equivalents of the quartz crystal (disregarding losses). The parallel capacity C5, by'so-me ways and means known in-the art, say, by the b-ridge scheme as shown comprising the coil L l. and

and L3, C3. The terminating resistance at the output end is designated by R2. In Fig. 2 which is an exemplified embodiment of the invention, thefquartz filter is inserted between two tubes. The parallel capacity of the quartz crystal Q also here has been neutralized,

though this is not specifically indicated. As to the rest, the references are the same as in Fig. l.

For changing the band width which is here effected by steps, though it could just as well be efiected continuously, there are provided four switches Sl-SG. ,1The parallel oscillation circuits L2-C2, and Lil-C3. are all tuned to the pass wave' of the crystal, and they distinguish themselves only by differences in" the ratio of inductance to capacity as indicated by the sizes chosen for the coils and the condensers. In a similar way, the various sizes of the terminating resistances are schematically represented in the drawing, In calculating the resistances R9 and R2, of course} the parallel internal resistance of the first tube or the input resistance of the second tube, as well Q as the natural damping resistances of the oscillation circuits imagined to be connected in parallel to the oscillation circuits must be considered, for it will be understood that the terminating resistances are composed of all of these resistances. In the left-hand position of the switches, the filter is set to the widest band, and

in the right-hand position to the narrowest band.

It shall now be shown in what way, by the aid of the quadripole or four-terminal network theory, it is possible to demonstrate that by the ways and means hereinbefore described, the band width is variable, while yet the characteristic form of the resonance curve is incidentally preserved.

In what follows, w1 w2 are to denote the limiting or cut-oi? frequencies of the range in which the quadripole damping becomes zero; Zm is the surge impedance at mean frequency m h/ l m and wg-w bthe relative width of .gap or interval in the sense of the theory underlying four-terminal networks. The said gap width as will be remembered, is

the band-pass of the filter in the theoretical Now, the demand is that the crystal, regardless of the band width, should stay unaltered so that LI and Cl would be constant. The result of this condition is (as shown by the first Equations 1 and 2) that the relation between Zm and b remains unvaried provided that the mean frequency mm, which, in the case of a receiver is equal to the I. F. stays constant. In other words, the surge impedance Zm for the mean frequency mm of the filter, when the gap Width b is doubled, is also doubled of necessity. Both these changes, as can be noted from Formulae 3 and 4 are brought about by quadrupling L2 and L3, while cutting C2 and C3 to one-fourth. The resonance frequency of the parallel resonance circuits remains unaltered no matter what the gap Width. This, in the first place, shows that width b is alterable by changing the ratio of L2 to C2, and of L3 to C3.

The fact that also the characteristic form of the resonance curve is preserved, no matter what the band width, in the presence of conditions as outlined, can be shown by the following equation of the resonance curve which is also calculable by the aid of the theory of four-terminal networks.

The following equation shows the absolute value of the relation between. output current to input where Bstands for the relative band width for different ordinate values in Fig. 3, that is to say, the actually existing band width referred to the mean frequency wm, contradistinct from the gap width b in the sense of the four-terminal network theory existing in the ideal case. For this filter, at the value of Z0 times the crest of the resonance curve, the actual band width B is equal to the gap width b according to the four-terminal network theory (marked by dots in Fig. 3).

The equation for the resonance curve shows that the characteristic form of the resonance curve, for different widths b is always the same, if the ratio n between the various terminating resistances RI and R2 to the surge impedance Zm of the filter stays the same. For then the shape of the resonance curve is solely a function of the relation between the actual band width B and the gap-Width b occurring twice in the root of the denominator. Thus, if the gap-Width b by varying the ratio of LC of the parallel resonance circuits is raised, say, from .1 to .3 (see Fig. 3) and if the terminating resistances are correspondingly changed, then, according to the equation of the resonance curve, the actual relative band width B, for all co-ordinate values, is trebled. The characteristic form is then the same as before inasmuch as, in line with what has been pointed out, it is determined only by the relation 11. between the terminating resistances RI and R2 and the surge impedance Zm. If the said ratio isequal to unity there results a resonance curve which approaches the rectangular form most 50 closely. But the resonance curve becomes triplehumped (three-wave) if n exceeds 1 (overmatching). But the crevasses, contradistinct to known quartz filters, do not grow as the band width grows since the form of the resonance curve stays unvaried with constant 11..

The output potential of the filter, for a constant input potential, is the same for all band widths. For it will be noted that for the mean frequency of the filter, the parallel resonance o0 circuits may be assumed to be non-existent, while the series resonance circuit may be conceived as being short-circuited. What then remains in the circuit arrangement Fig. 1 are merely the two terminating resistances which, no matter what the band widths, are equal to each other, with the result that at the output resistance there arises always the same output potential. Referring again to Fig. 2, if the first tube has a high internal resistance (pentode), then, in the presence of a constant grid alternating potential of this tube, the output potential of the filter increases practically in the same measure as the band Width because of the fact that, as the band width grows, the terminating resistances are 25 raised in the same ratio, and that the plate alternating voltage rises correspondingly.

What I claim is:

1. An electrical wave filter network comprising a crystal element in the series arm of a 1r-type filter, the shunt arms of which are each constituted by a selected one of a plurality of parallel resonant circuits, said circuits being tuned to the same operating frequency of the filter network but having difierent ratios of inductance to capacity.

2. An electrical wave filter network according to claim 1 wherein each shunt arm is adapted to have a selected one of a plurality of terminating impedances comiected therea-cross.

3. An electrical wave filter network comprising a crystal element which constitutes the series arm of a 1r-type filter, a plurality of resonant circuits tuned to the same operating frequency of the filter network arranged for selective connection to one side of the crystal element to constitute one of the shunt arms of the filter net work, corresponding parallel resonant circuits arranged forselective connection to the other side of the crystal element to constitute the other shunt arm, the circuits of each shunt arm having different ratios of inductance to capacity, and means for selectively connecting corresponding resonant circuits as shunt arms for the filter network whereby the band width of the network may be varied without altering the shape of the resonance curve.

4. An electrical wave filter network according to claim 3, wherein the parallel resonant circuits of each shunt arm are arranged in the order of their decreasing ratios of inductanceto capacity, the pass band of the filter being widest when the resonant circuits with the highest inductance to capacity ratio are operative, and the pass band being narrowest when the resonant circuits with the lowest inductance to capacity ratio are operative.

5. A crystal filter of variable band-pass width, comprising a crystal element which serves as the coupling element between a pair of vacuum tubes, a plurality of parallel resonant circuits tuned to a common frequency and arranged for selective connection between the anode and cathode of the first tube and between the signal grid and cathode of the second tube, corresponding pairs of said resonant circuits having a different inductance-capacity ratio from the others, a plurality of terminating resistances of different values arranged to be selectively shunted across each of the resonant circuits, and means for selectively switching in corresponding resistances and resonant circuits in cooperative relation with said crystal element.

'I-IANS GOERIN'G. 

